Engines are known that comprise two manifolds having branches that are configured to supply two gas streams to an intake port of each cylinder, the first manifold supplying a metered quantity of air within which the fuel to be burnt is dispersed and the second manifold supplying recirculated exhaust gases (EGR), the EGR gases serving as dilution gases.
Conventional engines do not achieve charge stratification because the two streams are well mixed on entering the cylinder. The EGR gases supplied through the second manifold during periods that the intake valve is closed, continue to enter and are stored in the intake port and in the branch of the first manifold. When the intake valve opens again, the intake charge first inducted into the cylinder comprises EGR gases that contain little oxygen yet have a high fuel content picked up from the walls of the wet intake port. This can be tolerated only if the combustion chamber is designed to produce good mixing of the charge during the compression stroke, as is the case in conventional engines. However, in a stratified charge engine, this storage of EGR gases in the branches of the first manifold must be avoided as such stratification would result in poor combustion quality.
In an engine of the type described initially, it is important to control the velocities of the two streams entering the combustion chamber such that there is minimum mixing between them during the intake and compression strokes in order to conserve the stratification up to the time of ignition. It is for this reason that in the case of swirling fluid motion in the combustion chamber, the linear velocity of the outer stream in the vortex should be faster than the linear velocity of the inner stream in the vortex such that their angular velocities are substantially equal in order to ensure minimum mixing.
In order to produce these velocities, a greater pressure difference must be applied along the branch of the faster stream than along the branch of the slower stream. Since the vacuum pressure in the intake port is common to both the branches, the vacuum pressure in the plenum chambers of the two manifolds upstream of the respective branches must be set unequally in order to provide the necessary pressure differences accordingly.
During the intake stroke of one of the cylinders, the suction in the associated intake branch would set up unequal pressures in the two plenum chambers causing balancing flows along the branches of the other cylinders not undergoing suction in the direction to equalise the pressures. These balancing flows are undesirable as the content of one stream would be replaced by the content of the other stream and the distinction in content between parallel streams would be lost.
WO96/10688 discloses an engine as described initially, wherein a non-return valve is arranged in each branch of the first manifold to permit gas flow in the branch only in the direction towards the intake valve. This achieves the required objective of maintaining the two streams separate at all times before they reach the combustion chamber but has the disadvantage that the non-return valves interfere with the aspiration of the engine under full load operation and reduce the maximum engine power output. To mitigate this effect, the non-return valves must be made as large as possible and this in turn leads to problems of high cost and difficulty in packaging.